1. Field of the Invention
This invention relates to the selective preparation of N-(aminoalkyl)piperazine and more particularly pertains to a direct liquid phase catalytic process for synthesizing predominantly non-polymer N-(aminoalkyl)piperazines.
2. Prior Art
There are several procedures described in the literature for directly preparing predominantly noncyclic alkyleneamines by the condensation reaction of an amino-alkanol compound and an alkylatable amine compound which do not require neutralization of the reaction product to obtain the desired salt-free polyamines.
For example, U.S. Pat. No. 3,714,259 to Lichtenwalter et al. describes a catalytic process for the preparation of polyethylene polyamines whereby an ethyleneamine compound and an ethanolamine compound are contacted in the presence of hydrogen and a hydrogenation catalyst comprised of oxides of nickel, copper, chromium, and like metals, in liquid phase at a temperature of 140.degree. C. to 170.degree. C. This procedure has been shown to require extended reaction times to provide acceptable conversions and yields of the polymeric polyethylene polyamines products.
Certain phosphoric acid compounds have been disclosed as effective as catalysts in promoting condensation reactions between some amines and tertiary aminoalkanols. However, the reaction conditions are relatively mild. For example, U.S. Pat. No. 3,121,115 to Meuly teaches a process for aminoalkylating certain amines having a replaceable amino hydrogen, particularly aromatic primary and secondary amines, which includes heating the amine compound with an N-tertiary aminoalkanol at from 150.degree. C. to 250.degree. C. in liquid phase with continuous water removal in the presence of a phosphoric acid compound. The disclosed process requires long reaction times, a disadvantage mentioned hereinbefore, and the use of an N-tertiary aminoalkanol. Thus there is a limitation on the products that can be formed.
We have now discovered an improved catalytic process whereby predominantly mono-N-aminoalkylated piperazine compounds can be produced from the selective dehydration of a primary or secondary aminoalkanol compound with a piperazine compound. The dehydration is somewhat selective to a single or higher oligomeric product. Thus, for example, the mono-N-aminoalkylated compound or the di-N,N'-aminoalkylated compound can be formed to the substantial exclusion of, for example, polyalkylene piperazine compounds. It also has been discovered that the condensation reaction can be carried out under rather severe processing conditions, such as temperatures above about 250.degree. C. in liquid phase without the expected decomposition of reactants and products with the attendant formation of high molecular weight polymers.